Liquefaction noncondensables

Texaco gasification is based on a combination of two process steps, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is cracked under relatively mild thermal conditions. This depolymerisation results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is used as a fuel in the process. The process is very comparable to the cracking of vacuum residues that originate from oil recycling processes. 

The dry gas is usually compressed before use. The level of compression depends on the application. A large fraction of the world s output of chlorine is consumed on site. The production of ethylene dichloride (EDC) is the single largest-volume use. The dry gas supply pressure then is determined by the needs of the EDC process. Merchant chlorine will be liquefied. The choice of compressor output pressure is then an economic/technical balance with the requirements of the liquefaction process. Because some of the impurities in the chlorine gas are noncondensables, a chlorine-containing tail gas always results. Some of this chlorine value can be recovered directly or in the form of various byproducts. A system is also required for safe disposal of any unrecovered chlorine as well as any released from the process during emergencies. 

The presence of noncondensable gases in the chlorine means that liquefaction is always incomplete. These gases leave the process and carry a certain amount of chlorine with them. This tail gas must be treated to remove the chlorine before it can be disposed of by venting it to the atmosphere. The chlorine value can be converted to a salable product such as bleach, HCl, or FeCls. It also can be recovered as elemental chlorine by absorption in a solvent followed by stripping. In some cases, it is simply absorbed in an alkaline medium and then treated for disposal. Section 9.1.9 covers the subject of tail gas handling. 

The two-phase process stream goes to a receiver that also serves as a phase separator. The liquefied chlorine leaves through a bottom connection. The uncondensed gas, containing some of the chlorine along with the noncondensable impurities, goes overhead. A valve on the outlet line maintains the desired pressure on the gas in the liquefier and also serves to control the discharge pressure of the chlorine compressor. The combination of the pressure and temperature on the process side of the liquefier determines the extent to which the chlorine can condense (see Achievable Degree of Liquefaction, below). 

A. Achievable Degree of Liquefaction. The major complication in the liquefaction of chlorine is the presence of noncondensable gases. These arise in several different... 

The noncondensables will tend to pass through liquefaction and will carry chlorine with them at approximately its vapor pressure at the process temperature. We should note here that CO2 is far less volatile than the other noncondensables considered here. More of it is likely to condense with the chlorine, and the analysis that follows here would require some modifrcation. Assuming that all of the noncondensables pass through liquefaction and that the mole fraction of chlorine in the gas is equal to its vapor pressure divided by the total operating pressure, we have... 

One of the advantages of membrane cells is the low hydrogen content of the cell gas. Dilution air may not be required, making the liquefaction process more efficient and the prevention of contamination by noncondensables more rewarding. 

Figure 9.40 outlines the process licensed by ELTECH Systems. Compressed gas at about 7 bar is cooled first with water and then with a refrigerant to—lOor—15°C. When this process follows a low-severity liquefaction system, this step alone can recover much of the chlorine. After removal of the liquid, the gas then passes up through an absorption column against cold CCI4. The noncondensable gases leave the absorption system under... 

In practice, the gas fed to liquefaction contains some noncondensable gas. Its sensible heat must be included in Eq. (43) and we have... 

The principal source of chlorine-containing gas in caustic-chlorine plants is the liquefaction step where noncondensables are vented from chlorine condensers as sniff gas containing 30 to 40% chlorine by weight. Dilute gas may be collected at other points in the operation this gas also requires purification before it can be vented to the atmosphere. A number of processes have been developed to recover the chlorine from the vent-gas streams, including its use for the manufacture of bleach. Where the demand for bleach does not justify this operation, a regenerative recovery system is neces.sary, and one of the simplest of these involves absorption in water. The absorption of chlorine gas in water is also an important step in the manufacture of certain types of wood pulp. In this application, the process is intended primarily to provide a source of concentrated bleaching solution however, design data which have been obtained for the absorption step are equally applicable to gas-purification or chlorine-recovery operations. 

A schematic diagram of the water-absorption process for recovering chlorine from caustic-chlorine process sniff gas developed by the Hooker Electrochemical Company (Anon., 1957) is. shown in Figure 6-21. In this process, chlorine-containing noncondensable gas from the liquefaction stage of chlorine manufacture is scrubbed countercurrently with water in a packed absorber. The resulting chlorine-free gas can be vented to the atmosphere, and the... 

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